Studies will be performed to ascertain 1) the radiosensitivity of hematopoietic stromal cells, the cumulative effect of successive exposures to x-irradiation separated by various time intervals, and the sensitivity of stromal cells to various chemotherapeutic agents. 2) The role of humoral factors on the rate of proliferation of hematopoietic stem cells. This will be done by studying the rate of regeneration of HSC in femora implanted into normal and irradiated parabionts. 3) The mechanism by which splenic erythropoiesis is increased to compensate for the decrease in erythropoiesis in the marrow of estrogen treated mice (which do not become anemic. 4) The effects of various androgenic and anabolic compounds on the rate of erythropoiesis; and on renal and extrarenal erythropoietin production in chronically uremic patients and rodents and 5 the kinetics of leukemic and normal HSC in REM mice transplanted with leukemic spleen cells.